The present invention relates to glass compositions for the manufacturing of lenses for optical and/or ophthalmic use, with an index of refraction (n.sub.d) higher than 1.88, a low dispersion (Abbe number greater than or equal to 29), and a density lower than approximately 4.g/cm.sup.3 .
For spectacle glasses of high correction glasses, the use of a glass with an index of refraction of approximately 1.9 permits, when compared to glasses with a lower index of refraction (1.7, for example) and equal power, appreciable reduction of the thickness at the edge (when it is a matter of negative power lenses) and in the center (when it is a matter of positive power lenses).
This constitutes an undeniable advantage for the wearer of the spectacles, in particular in case of high corrections (&gt;6 diopters). It is also known that increasing the index of refraction is generally accompanied by a deterioration of the other properties characteristic of the glass, and in particular the density increases. For a glass with high index to be able to retain the advantages of comfort mentioned above, it is necessary for its density not to be too high. Another disadvantage often associated with increasing the index is the increase of dispersion (or reduction of the Abbe number).
By reason of the above-mentioned constraints, and yet others regarding toxicity and cost, it is difficult to find satisfactory glass compositions with high index of refraction.